The invention relates to a method for the continuous electrolytic deposition of metals from aqueous solutions of metallic salts onto an elongated metal strip using a high flow speed of the electrolyte between an anode and a cathode in order to attain high current densities at relatively low voltages, especially for the electrolytic coating of a steel strip with non-ferrous metals, preferably with zinc, as well as the device for carrying out the method.
The use in electrolytic strip processing systems of soluble or non-soluble anodes which are employed in systems with a horizontal, vertical or radial strip feed in the processing part is known.
These anodes are usually adjustable and, to the extent that they are soluble anodes, they can be moved and replaced. The anodes cannot be moved relative to the direction of strip travel. The known disadvantages of such systems are caused, among other things, by the minimum space to be observed between the cathodic strip to be processed and the anodes. These spaces are necessary to prevent the strip from making contact with the anodes. The differences in traction which occur during the rolling of the strip, etc., frequently result in some corrugation of the strip edge and/or of the strip middle which in the case of horizontal and vertical processing systems necessitate strip spaces of at least approximately 10-15 mm. These spaces lead in the galvanic process to considerable voltage losses which result from the conductivity of the electrolyte used and from the space of the anode to the cathode, which necessitates considerable processing costs. In addition, the voltage losses in the electrolyte are converted into joulean heat, which necessitates greater cooling of the electrolyte than would otherwise be necessary.
The economic feasibility of electrolytic strip processing systems, e.g. the systems for coating steel strips with zinc, depends among other things on the possibility of achieving high current densities with acceptable voltages in the galvanic process. In addition to depending on the chemical composition of the electrolyte, the maximum obtainable current density depends on the thickness of the Nernst and Prandl border layers which determine the kinetics of the galvanic reaction. High flow speeds of the electrolyte are used in newer systems to reduce the thicknesses of these border layers. In the case of the specified spaces between the anode and the band to be coated, great amounts of electrolyte must be circulated to this end which necessitates the installation of appropriate pump means with a rather high consumption of energy.
The present invention is to a method of the type initially mentioned which will eliminate the disadvantages of the known methods, by means of which, particularly, when the strip is coated on only one side, a relatively very small distance between anode and cathode which can be set and regulated. Furthermore, low voltage bases in the electrolyte and a correspondingly lower development of heat are achieved. A very rapid exchange of the electrolyte in the slot between anode and cathode and a high current density are obtained and, by virtue of a very low friction on the strip to be coated, the industrial qualities of the strip are not adversely affected.